When treating semiconductor discs or wafers, the latter are, in the prior art, placed in wafer racks, which wafer racks are introduced into furnaces and a treatment at elevated temperature then takes place, with any type of process gas being supplied. In U.S. Pat. No. 5,407,449, which was assigned to ASM International N.V., a system is described in which the wafers are supplied in the conventional manner in cassettes. The wafers are removed from the cassettes by a central robot and transferred to the wafer racks for the furnace concerned. The wafer racks are fed to the furnace via a carousel. Each cluster consisting of a furnace and other treatment devices has its own loading and removal device.
When increasing the capacity of devices for the treatment of wafers it was first proposed in the prior art to place a number of such systems parallel to one another.
In a subsequent proposal, two furnaces were placed in one area and these furnaces were provided with wafers originating from cassettes with the aid of a single robot.
The problem when designing such systems is that the robots for transferring wafers from cassettes to wafer racks have to be so dimensioned that the time for transfer of the wafers does not constitute a limiting factor, even for the shortest treatment time in the furnaces. One solution to this problem comprises positioning a number of robots above one another for simultaneous loading of a wafer rack.
In practice, however, it is found that the capacity of such robots is seldom used to the full. Moreover, the installation costs for a large number of robots are appreciable, as is the space requirement.
U.S. Pat. No. 5,178,639A discloses a vertical heat-treating apparatus, comprising a number of adjacent furnaces. Supply of wafer filled wafer racks is realized through a single supply rail along which the displacement device is transferable between each of the furnaces and a central loading/unloading area. The displacement device comprises a simple pedestal for the wafer racks.